This invention relates to a substrate assembly for a fluorescent or phosphorescent display panel.
A fluorescent or phosphorescent display panel, namely, a luminescent display panel as called in general herein, comprises a substrate assembly comprising, in turn, a substrate of an electrically insulating material, such as glass or ceramics, a plurality of electroconductive electrodes on the substrate, and segmented electrodes on the electroconductive electrodes, respectively. As will become clear later, each segmented electrode may consist of a graphite intermediate layer disposed on each electroconductive electrode and a segmented layer placed on the intermediate layer. A mass of a luminescent material is deposited on each segmented layer or electrode. Each segmented electrode is generally placed in an indent formed in the substrate in a corresponding geometrical configuration with the luminescent mass disposed also therein so as not to protrude outwardly of the general surface of the substrate. The expression "on the substrate" should therefore be understood to mean that the segmented electrodes and luminescent masses do not necessarily protrude from the general substrate surface. Each segmented electrode and the luminescent mass deposited thereon form a display electrode. The substrate may be a composite substrate having two or more substrate layers.
For a multi-digit luminescent display panel, such a substrate assembly is sealed in a vacuum envelope together with a grid for each digit and at least one hot cathode for a plurality of digits.
Alternatively, a cup-shaped cover glass sheet is hermetically sealed at its periphery to such a substrate assembly after the grids and cathode or cathodes are preliminarily attached to the latter. A substrate assembly therefore further comprises supports for the grids and cathode. A plurality of electroconductive leads are disposed on the substrate to provide electric connections to the electroconductive electrodes and the grid and cathode supports.
It has been the practice to manufacture the electroconductive electrodes and leads by firing prints formed, on a glass or a ceramic substrate, of silver or silver-palladium paste at about 600.degree. C. Thereafter, a film of an electrically insulating vitreous material is put by screen printing on the substrate and the electroconductive electrodes and leads with openings provided through the print of film at positions where the segmented electrodes should subsequently be formed. The print of film is fired at about 600.degree. C to provide an insulating film. The openings thereby become indents formed in a composite substrate. The graphite intermediate layers of the segmented electrodes are formed by disposing in the respective indents, by printing, masses of a paste-like composition consisting essentially of graphite powder and a low melting point vitreous material that includes lead oxide. The printed masses are fired at about 600.degree. C. The segmented layers are made by similarly firing prints of the paste-like composition. Alternatively, the segmented layers may be made by firing prints of silver or other metal paste at about 600.degree. C. The vitreous material includes lead oxide because this lowers the melting point and renders the composition suitable to printing.
The graphite intermediate layers increase the brightness of luminescence of the luminescent masses when used together with a very small amount of the vitreous material. They, however, do not tenaciously adhere to the underlying electroconductive electrodes and particularly to the insulating film and come off together with the luminescent masses when it is desired to remove those luminescent masses for reuse of the substrate assembly at least one of which is inadvertently wrongly deposited on the graphite layer. The adhesion is enhanced by increasing the content of the vitreous material in the composition. This means an increase in the content of lead in the graphite layer. The lead, however, adversely affects the electron emissivity of the hot cathode and consequently reduces the brightness of the luminescence.